The present invention is generally directed to transmitter systems capable of switched operation between a single transmit path and two transmit paths with weakly correlated signals in each path. More specifically, the present invention is directed to the field of wireless local area network transmitter systems. In particular, the present invention applies to wireless local area network access points using orthogonal frequency division multiplexing with two antenna transmit maximum ratio combining.
Wireless local area networks typically include an access point that wirelessly communicates with one or more clients. The access point also generally provides a wired connection for communicating with the network. Thus, the access point provides an interface between the wired network and the remote or wireless clients.
Wireless local area network access point operation is often defined by the Institute of Electrical and Electronics Engineers (IEEE) 802.11a, 802.11b and/or 802.11g standards. A number of proprietary systems are also known to exist. In the standards based systems, data is transferred between the access point and the clients using radio frequency (RF) channels or carriers in the 2.4 Gigahertz Industry Scientific and Medical and the Universal Networking Information Infrastructure 5 Gigahertz bands. The radio frequency carriers are modulated using various digital modulation techniques that include; for example, direct sequence spread spectrum and orthogonal frequency division multiplexing modulation techniques.
Although an access point can support a radio link with many clients, an access points must also be capable of a “unicast” transmission intended for a single client. Moreover, it is desirable to optimize the unicast transmission for the intended client to improve the robustness of the data transfer between the access point and the intended client. A unicast transmission can be optimized in a wireless local area network using two antenna transmit maximum ratio combining. Two antenna transmit maximum ratio combining seeks to overcome the signal to noise degradation at the intended client caused by multiple reflections of the signal arriving at the client which combine or add in a destructive manner. Two antenna transmit maximum ratio combining is accomplished in the access point by splitting a transmitter signal into two separate transmission paths. The signals in each transmit path are then weighted in amplitude and/or phase such that the signals recombine constructively at the intended client. Weighting can be frequency or channel specific, meaning that different weights are used for different channels within a frequency band.
As mentioned, wireless local area network operation also requires the access point to make “multicast” transmissions that are intended for multiple clients. Thus, an access point transmitter must be capable of switching between unicast and multicast transmission types. Generally, multicast transmissions use a single transmit path and a single antenna element, while unicast transmissions employing two antenna transmit maximum ratio combining require multiple transmit paths, each path having a separate respective power amplifier and antenna element.
FIGS. 1A and 1B show a transmitter system 10 that is capable of switched operation between unicast and multicast transmissions. Transmitter system 10 generally comprises a two antenna array 11 having a first antenna element 12 and a second antenna element 14. Transmitter system 10 further comprises first and second power amplifiers 16, 18, both of which have a gain G. More specifically, transmitter system 10 has a first transmit path defined by power amplifier 16 and antenna element 12 and a second transmit path defined by power amplifier 18 and antenna element 14.
As shown in FIG. 1A, and in the unicast operating mode, the input power Pin is distributed between two signals S1 and S2. The power associated with S1 and S2 are given by the expressions a·Pin and (1−a)·Pin, respectively. The distribution of the input power Pin between signals S1 and S2 is not necessarily even, and the exact power distribution is accounted for in the value of a, where 0<a≦1. For example, if the value of a is allowed to equal 1, then all of the input power is distributed to signal S1 and no power is distributed to signal S2. This is the condition for multicast operation, shown in FIG. 1B. Thus, multicast operation for transmitter system 10 is the condition where a=1, and all of the input power Pin is concentrated in signal S1.
Viewed in this manner, four distinct operating conditions exist which are relevant to the design and/or selection of power amplifiers 16 and 18. These four operating conditions are as follows: (1) unicast mode where 0<a<0.5, (2) unicast mode where a=0.5, (3) unicast mode where 0.5<a<1, and, as previously described, (4) multicast mode, where a=1.
In the first operating condition, the input power Pin is split unevenly between signals S1 and S2, and the majority of the power is delivered to power amplifier 18. If a approaches zero, nearly all of the input power Pin will be distributed to power amplifier 18 and only a small fraction of the input power Pin will be distributed to power amplifier 16. In the third and fourth operating conditions, when a either approaches or is equal to unity, nearly all or all of the input power Pin will be distributed to power amplifier 16 and only a small fraction or none of the input power Pin will be distributed to power amplifier 18. Under these operation conditions, it is possible for the entire radiated power to be delivered by a single amplifier, e.g., either power amplifier 16 or 18; and thus, to allow for all operating conditions, both power amplifiers 16, 18 must be designed and/or selected such that they are capable of delivering the entire radiated power Pradiated=G·Pin.
Although each power amplifier 16, 18 must be designed and/or selected to deliver a maximum power level equal to Pradiated=G·Pin, it is clear that under most conditions one or both power amplifiers 16, 18 will be operating significantly below that level. This leads to inherently inefficient operation of the power amplifiers 16, 18.
Thus, there exists a need for a transmit system that provides constant power amplifier loading between single and dual signal operation. Moreover, there is a need for a transmit system that evenly distributes single or dual signals between power amplifier components independent of operating conditions. Further, there is a need for a transmit system that provides power amplifier load balancing independent of single or dual signal operation.